MLB calls this a "grand slam single." How is that possible?
In a tie game in extra innings and the home team is at bat, the game is over as soon as the winning run is scored. So even though the batter hit a homerun and the bases are loaded (hence a "grand slam"), once the runner crosses home plate and the batter reaches first, there's no reason for the other runners continuing. You can see the team celebrating with the batter in the field and because he didn't touch all the bases, so it was officially scored as a single.
"grand slam single" isn't an official term. As the batter hit a home run with the bases loaded, he would normally be allowed to touch all the bases and have all 4 runners score (7.02(g)(3)).
But after the ball was struck, teammates came onto the field to celebrate and prevented him from continuing to second base. You can see a quick shot Ventura waving to keep them away so he could finish, but they didn't depart.
Per the approved ruling, if the batter does something to cause him to be out (in this case 5.09(b)(2), abandoning an effort to touch the next base after having touched first base), then the game is over after the winning run scores. The batter is credited with a single and (in this case) one RBI.